1. Field of the Invention
The subject application relates to a handheld device and a planar antenna thereof. More particularly, the handheld device of the subject application can operate at multiple central frequencies by controlling short points of the planar antenna thereof.
2. Descriptions of the Related Art
With rapid development of wireless communications, demands on wireless communications also become increasingly higher. Among various wireless communication products, portable handheld devices are the most demanded by people nowadays. In order to satisfy the demands of modern people on thinner and lighter handheld devices, various antennas for the handheld devices have also been developed rapidly. Currently, built-in antennas that are hidden in the handheld devices have become the mainstream in the market, and among such built-in antennas, planar antennas are the most favored. This is mainly because that a planar antenna occupies a significantly reduced space in a handheld device. Accordingly, many researches and inventions related to planar antennas have occurred at an amazing speed.
As handheld devices having the function of supporting operation in a plurality of frequency bands are preferred by the users, antennas operable at multiple central frequencies have arisen. For a conventional planar antenna, in order to operate at multiple central frequencies, at least two radiators shall be provided as resonators for the central frequencies. Therefore, the structure of the conventional planar antenna is bound to be changed. In other words, in order for a handheld device to operate at more central frequencies, the antenna of the handheld device must be increased in size. Consequently, the handheld device is made to have an increased volume, which is in contradiction to the trend of miniaturization of handheld devices.
On the other hand, once a central frequency at which the conventional planar antenna is to operate is determined, the structure of the planar antenna is fixed. Therefore, it is difficult to change the central operating frequency without changing the antenna structure. For example, when a planar antenna operating at a central frequency of 900 MHz is to operate at a central frequency of 1800 MHz instead, the structure of the planar antenna has to be re-designed in order to achieve this purpose. However, it seems difficult for a user to change the central frequency at which the antenna can operate by changing the antenna structure. Therefore, for antenna designers, how to allow the handheld devices to support operation at a plurality of frequencies in a more convenient way will become an important issue in the future.
In view of this, an urgent need exists in the art to provide a solution that can change a central operating frequency of a planar antenna and provide a better communication quality without increasing the overall volume of the antenna.